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Yonezawa T.,Tohoku University | Watanabe M.,Tohoku University | Hashimoto A.,Kobe Material Testing Laboratory Co
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2015

Primary water stress corrosion cracking growth rates (PWSCCGRs) in highly cold-worked thermally treated (TT) Alloy 690 have been recently reported as exhibiting significant heat-to-heat variability. Authors hypothesized that these significant differences could be due to the metallurgical characteristics of each heat. In order to confirm this hypothesis, the effect of fundamental metallurgical characteristics on PWSCCGR measurements in cold-worked TT Alloy 690 has been investigated. The following new observations were made in this study: (1) Microcracks and voids were observed in or near eutectic crystals of grain boundary (GB) M23C6 carbides (primary carbides) after cold rolling, but were not observed before cold rolling. These primary carbides with microcracks and voids were observed in both lightly forged and as-cast and cold-rolled TT Alloy 690 (heat A) as well as in a cold-rolled TT Alloy 690 (heat Y) that simulated the chemical composition and carbide banded structure of the material previously tested by Paraventi and Moshier. However, this was not observed in precipitated (secondary) M23C6 GB carbides in heavily forged and cold-rolled TT Alloy 690 heat A and a cold-rolled commercial TT Alloy 690. (2) From microstructural analyses carried out on the various TT Alloy 690 test materials before and after cold rolling, the amount of eutectic crystals (primary carbides and nitrides) M23C6 and TiN depended on the chemical composition. In particular, the amount of M23C6 depended on the fabrication process. Microcracks and voids in or near the M23C6 and TiN precipitates were generated by the cold rolling process. (3) The PWSCCGRs observed in TT Alloy 690 were different for each heat and fabrication process. The PWSCCGR decreased with increasing Vickers hardness of each heat. However, for the same heats and fabrication processes, the PWSCCGR increased with increasing Vickers hardness due to cold work. Thus, the PWSCCGR must be affected not only by hardness (or equivalently the cold working ratio) but also by grain size, microcracks, and voids of primary M23C6 carbides, etc., which in turn depend on chemical composition and the fabrication process. © 2015, The Author(s).

Yonezawa T.,Tohoku University | Suzuki K.,Tohoku University | Ooki S.,Tokyo Electric Power Company | Hashimoto A.,Kobe Material Testing Laboratory Co.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2013

In order to establish more reliable formulae for calculating stacking fault energies (SFE) from the chemical compositions of austenitic stainless steels, SFE values were measured for 54 laboratory-melted heats and 2 commercial heats. The results were checked against those of a firstprinciple, atomistic calculation approach. More than ∼20,000 data points for the width and angle of the Burgers vectors were determined from dark-field images of isolated extended dislocations in 56 heats of austenitic stainless steel using weak electron beams with g-3g diffraction conditions. Based on these numerous observations and on fundamental thermodynamic analyses, it is concluded that the SFE values for austenitic stainless steels are changed not only by chemical composition but also by heat treatment. In this paper, new formulae for calculating SFE values from the chemical compositions in three different heat treatment conditions have been proposed for austenitic stainless steels within given limited chemical composition ranges. In these formulae, the SFE values are calculated from the nickel, chromium, molybdenum, silicon, manganese, nitrogen, and carbon contents for the each heat treatment condition. The three heat treatment conditions studied were water cooling after solution heat treating (SHTWC), furnace cooling after solution heat treating, and aging after SHTWC. © The Minerals, Metals & Materials Society and ASM International 2013.

Arioka K.,Institute of Nuclear Safety System Ltd. | Yamada T.,Institute of Nuclear Safety System Ltd. | Miyamoto T.,Kobe Material Testing Laboratory Co. | Aoki M.,Institute of Nuclear Safety System Ltd.
Corrosion | Year: 2014

The purpose of this research was to compare the stress corrosion cracking (SCC) resistance of materials used at the present time for steam generator (SG) tubing in pressurized water reactor (PWR) primary water. Our results in PWR primary water for 20% cold-worked (CW) Alloy 800 (UNS N08800) are compared with already published data for SCC growth from 20%CW Alloy thermally treated (TT)690 (UNS N06690), 20%CW Alloy mill-annealed (MA)600 (UNS N06600), and 20%CW austenitic stainless steels. The second purpose was to examine the dependence of SCC growth on nickel and chromium in PWR primary water; the objective was to obtain the basic knowledge to understand SCC behavior of SG tubing materials. The third objective was to understand whether accelerated testing at higher temperatures is appropriate for predicting SCC initiation and growth at lower temperatures. For these objectives, SCC growth was measured in PWR primary water at 290, 320, 330, 340, and 360°C under static load conditions. Tests were performed using 0.5T compact tension-type specimens using laboratory-melted 20%CW Alloy 800 (UNS N08800, CW800NG) and 20%CW X%Ni-16%CW-Fe alloys in the range of nickel concentration between 16% to 60%. Four important patterns were observed. First, excellent SCC growth resistance was observed for 20%CW 800NG at 320°C and 340°C; second, significant effect of nickel on IGSCC resistance was observed at 340°C and 360°C. The rate of IGSCC growth decreases with increasing nickel concentration in the range of nickel concentration between 10% to 25% nickel; then, the rate of IGSCC increases with increasing nickel concentration in the range of nickel content between 50% to 76%. This trend is quite similar to the results reported by Coriou and Staehle tested in dearated pure water at 350°C. No significant dependence of IGSCC in pure water at 320°C and 290°C was observed. The change in SCC growth dependence on nickel concentration suggested that the main rate-limiting processes on IGSCC growth seems to change between 320°C to 340°C. Third, significant beneficial effects of chromium in alloys were observed at 320°C. However, no beneficial effect of chromium addition in alloys was observed at 360°C. Finally, peak temperatures in growth rate of IGSCC were observed in almost all test materials except for 20%CW Alloy 600. The most important engineering meaning of the complicated temperature dependence with peak is that the mechanism of IGSCC growth at higher temperature is different from that at operating temperature. Furthermore, the order of SCC resistance at higher temperature is not the same at operating temperature. This means that we should pay careful attention to assess SCC from accelerated testing at higher temperatures. © 2014, NACE International.

Arioka K.,Institute of Nuclear Safety Systems Inc. | Iijima Y.,Tohoku University | Miyamoto T.,Kobe Material Testing Laboratory Co.
Philosophical Magazine | Year: 2015

The diffusion coefficient of nickel in cold-worked carbon steel was determined with the diffusion couple method in the temperature range between 320 and 450 °C. Diffusion couple was prepared by electro-less nickel plating on the surface of a 20% cold-worked carbon steel. The growth in width of the interdiffusion zone was proportional to the square root of diffusion time to 12,000 h. The diffusion coefficient (DNi) of nickel in cold-worked carbon steel was determined by extrapolating the concentration-dependent interdiffusion coefficient to 0% of nickel. The temperature dependence of DNi is expressed by DNi = (4.5 + 5.7/-2.5) × 10-11 exp (-146 ± 4 kJ mol-1/RT) m2s-1. The value of DNi at 320 °C is four orders of magnitude higher than the lattice diffusion coefficient of nickel in iron. The activation energy 146 kJ mol-1 is 54% of the activation energy 270.4 kJ mol-1 for lattice diffusion of nickel in the ferromagnetic state iron. © 2015 © 2015 Taylor & Francis.

Maekawa A.,Japan Institute of Nuclear Safety System | Oumaya T.,Kansai Electric Power Co. | Noda M.,Japan Institute of Nuclear Safety System | Takahashi S.,Japan Institute of Nuclear Safety System | Saito T.,Kobe Material Testing Laboratory Co.
Materials Science Forum | Year: 2010

This paper describes residual stress measurements and analysis of austenitic stainless steel pipe with a butt-welded joint. The measurements were done with neutron diffraction and strain gauge techniques. The measured results had typical characteristics of butt-welded pipe regarding both the decline of stress along the axial direction and the bending distribution of axial stress along the radial direction. The measured residual stress distribution by neutron diffraction was shifted more to the tensile side than that by the finite element method simulation. However, the measured radial and axial strains, except for the hoop strain determined by neutron diffraction, coincided well with analysis strains. The hoop strain was actually equivalent strain converted by a correction method because a different lattice plane had to be used to measure hoop strain. This might be one reason why the difference occurred. Therefore, future study of the correction method would be desirable. © (2010) Trans Tech Publications.

Yamabe J.,Kyushu University | Awane T.,Kyushu University | Awane T.,Kobe Material Testing Laboratory Co. | Matsuoka S.,Kyushu University
International Journal of Hydrogen Energy | Year: 2015

The apparent hydrogen diffusivity and the saturated hydrogen content of Cr-Mo and Ni-Cr-Mo steels were determined with high-pressure hydrogen gas. Surface effects on hydrogen entry and exit were also investigated by using palladium-coated samples and by diffusion analysis using the finite-element method. Hydrogen contents of hydrogen-exposed cylindrical specimens of various sizes were measured by means of gas chromatography-mass spectrometry to obtain the saturated hydrogen content. The diffusivity was determined by fitting the solution of a diffusion equation to the experimental hydrogen contents determined by desorption at various constant temperatures. In the specimens examined, surface effects were significant at room temperature. The temperature dependences of the diffusivity were reasonably consistent with reference data mainly measured with electrochemical charging. These results were interpreted in terms of hydrogen trapping. Ordinary electrochemical charging represents a more severe condition than exposure to high-pressure hydrogen, for example, at 100MPa. © 2015 Chinese Medical Association Production.

Matsunaga H.,Kyushu University | Yoshikawa M.,Kyushu University | Yoshikawa M.,Kobe Material Testing Laboratory Co. | Kondo R.,Kyushu University | And 2 more authors.
International Journal of Hydrogen Energy | Year: 2015

Abstract Slow strain rate tensile (SSRT) tests were performed using smooth specimens of two types of steels, the Cr-Mo steel, JIS-SCM435, which has a tempered, martensitic microstructure, and the carbon steel, JIS-SM490B, which has a ferrite/pearlite microstructure. The tests were carried out in nitrogen gas and hydrogen gas, under a pressure of 115 MPa at three different temperatures: 233 K, room temperature and 393 K. In nitrogen gas, these steels exhibited the so-called cup-and-cone fracture at every temperature. In contrast, surface cracking led to a marked reduction in ductility in both steels in hydrogen gas. Nonetheless, even in hydrogen gas, JIS-SCM435 exhibited some reduction of area after the stress-displacement curve reached the tensile strength (TS), whereas JIS-SM490B demonstrated little, if any, necking in hydrogen gas. In addition, tension-compression fatigue testing at room temperature revealed that these steels show no noticeable degradation in fatigue strengths in hydrogen gas, especially in the relatively long-life regime. Considering that there was little or no hydrogen-induced degradation in either the TS or the fatigue strength in JIS-SCM435, it is suggested that the JIS-SCM435 is eligible for safety factor-based fatigue limit design for hydrogen service under pressures up to 115 MPa. Copyright © 2015, Hydrogen Energy Publications, LLC.

Yamabe J.,Kyushu University | Awane T.,Kyushu University | Awane T.,Kobe Material Testing Laboratory Co. | Matsuoka S.,Kyushu University
International Journal of Hydrogen Energy | Year: 2015

This study presents a precise hydrogen-barrier mechanism of a newly developed three-layer (alumina/aluminum/ferro-aluminum) aluminum-based coating in high-pressure gaseous hydrogen. After exposure to high-pressure gaseous hydrogen, the hydrogen content of the specimen with a palladium-sputtered aluminum-based coating was the same as that of the specimen with aluminum-based coating, but without palladium. Furthermore, the hydrogen content of the coated specimens increased with a decrease in the specimen size. These results indicate that the hydrogen entered by a diffusion-controlled process. The effective diffusivity of the coated specimen was approximately one thousandth of that of base steel (type 304 stainless). Such excellent resistance could not be obtained with a two-layer coating (alumina/ferro-aluminum). Analysis of local hydrogen concentrations by secondary ion mass spectroscopy demonstrated that the extremely low effective hydrogen diffusivity of the three-layer-coated specimen was attributed to hydrogen trapping at the aluminum-ferro-aluminum interface, and not to the hydrogen-entry obstruction by the aluminum layer. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Epoxy resin attached to a fatigue fracture surface of Ti-Al-Nb alloy was removed using a removal method for hardly soluble organic material attached to metallic material, which has been developed by the author. In the removal method process, the epoxy resin attached to the fracture surface was treated with an organic solvent, 'tetrahydrofuran', and cold concentrated sulfuric acid of nearly 100% purity. After the epoxy resin was removed from the fracture surface with the removal method, damage of the microscopic feature of the fracture surface was investigated using a scanning electron microscope (SEM). For the first time, the degree of the removal of the epoxy resin with the method was investigated by energy dispersive X-ray spectroscopy (EDS) in this research. After the removal, no damage of the fracture surface was found with SEM observation. In addition, C K derived from the epoxy resin was not detected with the EDS after removal. The result of the EDS analysis clarified that the epoxy resin was completely removed with the removal method. © 2013 The Author.

Tsurui T.,Kobe Material Testing Laboratory Co.
Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B | Year: 2013

Specimens with a gage section diameter of 1 mm and 6 mm were prepared from 12 Cr steel used over a long period in a power plant, and were compared for the difference of creep rupture strength depending on the gage-section diameter. Specimens of 2.25 Cr steel with a gage section diameter of 1 mm and 6 mm were also compared for the difference of creep rupture strength. As a result, 12 Cr and 2.25 Cr steels were found to have different size dependence of creep rupture strength, and the difference was studied on the effect of specimen diameter and thickness of oxidation film. ©2013 The Japan Society of Mechanical Engineers.

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